Scroll down a looooong ways until you get to Trivia.
If you don't want to...
Despite Ghost-type moves running off the Pokémon's Attack prior to Generation IV, Shadow Ball has always lowered Special Defense.
Ah I see, so they changed its attack type in gen 4 lol, why did my Mightyena then do so little damage? I know it can't just be because of nature, bug maybe?